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Effect of Abrasive Machining on the Electrical Properties Cu86mn12ni2 Alloy Shunts

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Effect of Abrasive Machining on the Electrical Properties Cu86mn12ni2 Alloy Shunts materials Article Effect of Abrasive Machining on the Electrical Properties Cu86Mn12Ni2 Alloy Shunts Siti Nabilah Misti 1,*, Martin Birkett 1 ID , Roger Penlington 1 ID and David Bell 2 1 Department of Mechanical and Construction Engineering, Northumbria University, Newcastle upon Tyne NE1 8ST, UK; [email protected] (M.B.); [email protected] (R.P.) 2 Vice Chancellors Office, Teesside University, Middlesbrough TS1 3BA, UK; [email protected] * Correspondence: [email protected]; Tel.: +44-191-2273763 Received: 10 May 2017; Accepted: 21 July 2017; Published: 29 July 2017 Abstract: This paper studies the effect of abrasive trimming on the electrical properties of Cu86Mn12Ni2 Manganin alloy shunt resistors. A precision abrasive trimming system for fine tuning the resistance tolerance of high current Manganin shunt resistors is proposed. The system is shown to be capable of reducing the resistance tolerance of 100 µW shunts from their standard value of ±5% to <±1% by removing controlled amounts of Manganin material using a square cut trim geometry. The temperature coefficient of resistance (TCR), high current, and high temperature performance of the trimmed shunts was compared to that of untrimmed parts to determine if trimming had any detrimental effect on these key electrical performance parameters of the device. It was shown that the TCR value was reduced following trimming with typical results of +106 ppm/◦C and +93 ppm/◦C for untrimmed and trimmed parts respectively. When subjected to a high current of 200 A the trimmed parts showed a slight increase in temperature rise to 203 ◦C, as compared to 194 ◦C for the untrimmed parts, but both had significant temporary increases in resistance of up to 1.3 µW. The results for resistance change following high temperature storage at 200 ◦C for 168 h were also significant for both untrimmed and trimmed parts with shifts of 1.85% and 2.29% respectively and these results were related to surface oxidation of the Manganin alloy which was accelerated for the freshly exposed surfaces of the trimmed part. Keywords: abrasive machining; Manganin alloy; shunt resistor 1. Introduction Shunt resistors have been used as ammeters to measure the flow of electrical current for several decades and offer the advantages of lower cost, low power loss, high stability, and precision of electric resistance across a wide temperature range. This is significant when compared to other current sensing methods such as current transformers, Hall sensors, and Rogowski coils [1]. Shunt resistors are typically manufactured from a Manganin (Cu86Mn12Ni2 wt %) alloy element which is an electron beam welded to two low resistivity copper terminations to permit accurate electrical measurement. Manganin is a commercially available alloy of resistivity 48.2 × 10−8 Wm [2] and low Temperature Coefficient of Resistance (TCR) of ±15 ppm/◦C[3], offering low and stable resistance across a wide temperature range. It also possesses an extremely low thermal electromotive force (EMF) of 0.1 µV/K at 20 ◦C and excellent long-term stability of electrical resistance. One recent important use of Manganin shunts is in single phase smart energy meters to measure the flow of electrical current. Due to its relatively low cost, the shunt resistor is the preferred current sensing method in this application where it must maintain a stable and repeatable resistance value over a wide current range of 0 to 100 A, across an operating temperature of −25 to +55 ◦C and relative Materials 2017, 10, 876; doi:10.3390/ma10080876 www.mdpi.com/journal/materials Materials 2017, 10, 876 2 of 10 Materialsair humidity 2017, 10 of, 876 30 to 100% [4]. In order to minimize overall power consumption of the smart energy2 of 10 meter, the room temperature resistance value of the shunt must be as low as possible and typically in the range 100 μΩ to 10 mΩ [5]. Although this low resistance requirement reduces energy usage, it in the range 100 µW to 10 mW [5]. Although this low resistance requirement reduces energy usage, also causes two significant issues. Firstly, the voltage drop that must be sensed across the shunt is it also causes two significant issues. Firstly, the voltage drop that must be sensed across the shunt substantially reduced, this in turn leads to a requirement to incorporate more accurate measuring is substantially reduced, this in turn leads to a requirement to incorporate more accurate measuring equipment into the energy meter. The second issue, which is the focus of this current work, is that it equipment into the energy meter. The second issue, which is the focus of this current work, is that it is is inherently difficult to manufacture shunt resistors in this low resistance range to the required inherently difficult to manufacture shunt resistors in this low resistance range to the required precision precision and at a reasonable cost. Typical resistance accuracy of commercially available shunts and at a reasonable cost. Typical resistance accuracy of commercially available shunts suitable in this suitable in this application is 100 μΩ to a tolerance of ±5% [6]. When used to measure current flow in application is 100 µW to a tolerance of ±5% [6]. When used to measure current flow in the smart the smart energy meter, this tolerance can result in power usage being either over or under calculated energy meter, this tolerance can result in power usage being either over or under calculated by up to by up to 5%. The current method used to reduce this inaccuracy is to calibrate the overall performance 5%. The current method used to reduce this inaccuracy is to calibrate the overall performance of the of the assembled meter. This is not always accurate across the full operating conditions and involves assembled meter. This is not always accurate across the full operating conditions and involves the the addition of compensation software which increases the overall cost of the meter. A more efficient addition of compensation software which increases the overall cost of the meter. A more efficient way way to improve the accuracy of the smart energy meter would be to reduce the resistance tolerance to improve the accuracy of the smart energy meter would be to reduce the resistance tolerance of the of the Manganin shunt resistor itself to less than ±5%. Manganin shunt resistor itself to less than ±5%. There are a number of well-established methods of improving the resistance accuracy of thin There are a number of well-established methods of improving the resistance accuracy of thin and and thick film discrete resistors by removing resistive material and adjusting the geometry of the thick film discrete resistors by removing resistive material and adjusting the geometry of the element element to increase its resistance. The most popular of these techniques are laser trimming, abrasive to increase its resistance. The most popular of these techniques are laser trimming, abrasive trimming trimming with a wheel, or abrasive particles and machining [7]. In the majority of processes, the with a wheel, or abrasive particles and machining [7]. In the majority of processes, the material is material is removed in single or multiple lines cut perpendicular to the flow of current to give high removed in single or multiple lines cut perpendicular to the flow of current to give high rates of rates of resistance change or in parallel with the current flow to give slower rates of change [8]. resistance change or in parallel with the current flow to give slower rates of change [8]. Although these Although these methods have yielded excellent results when adjusting the resistance tolerance of methods have yielded excellent results when adjusting the resistance tolerance of thin and thick film thin and thick film resistors, there has been limited application in the area of bulk metal alloy shunts. resistors, there has been limited application in the area of bulk metal alloy shunts. Recent work by the authors has highlighted the potential of a new trimming approach using Recent work by the authors has highlighted the potential of a new trimming approach using different geometry cuts to adjust the resistance tolerance of shunt resistors [9]. This current study will different geometry cuts to adjust the resistance tolerance of shunt resistors [9]. This current study will investigate the effect of abrasive machining using a square cut geometry on the principal electrical investigate the effect of abrasive machining using a square cut geometry on the principal electrical properties of 100 μΩ Manganin alloy shunts. properties of 100 µW Manganin alloy shunts. 2. Experimen Experimentaltal Procedures Procedures 2.1. Materials Materials All samples used in this investigationinvestigation werewere constructedconstructed fromfrom aa 1515× × 5 ×× 33 mm thick Manganin alloy element which was electronelectron beambeam weldedwelded toto twotwo 22.522.5× × 15 × 33 mm mm thick thick low low resistance resistance copper copper terminationsterminations.. This This produc produceses a a 100 100 μµΩW ± 5%,5%, 3 3 W W rated rated shunt shunt resistor resistor as shown in Figure1 1.. TheThe resistance values values of of a a batch batch of of 50 50 shunts shunts were were measured measured to tofind find a sam a sampleple of 20 of parts 20 parts in the in range the range 95– 9995–99 μΩ µwhichW which were were suitable suitable for forabrasive abrasive trimming trimming to a to target a target value value of 100 of 100μΩµ. W. Figure 1. ManganinManganin Alloy Alloy Shunt Shunt Construction Construction (dimensions in mm). 2.2. Abrasive Trimming System The experimental setup for the concurrent trimming process used in this study is shown in Figure 2. The shunt samples were trimmed using a Buehler Isomet 5000 linear precision saw fitted with a 178 mm diameter, 0.8 mm thick, rubber bonded silicon carbide (R/SiC) AcuThin cutting disc.
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